drm/xe/pxp: Add GSC session initialization support

A session is initialized (i.e. started) by sending a message to the GSC.
The initialization will be triggered when a user opts-in to using PXP;
the interface for that is coming in a follow-up patch in the series.

v2: clean up error messages, use new ARB define (John)

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: John Harrison <John.C.Harrison@Intel.com>
Reviewed-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250129174140.948829-7-daniele.ceraolospurio@intel.com